The distant object that some astronomers think could be the Solar System's 10th planet may have a moon.

The new planetary candidate, which has been named Sedna, rotates more slowly on itself than expected, suggesting it may have a sattelite orbiting it.

One of the scientists who found Sedna has been giving further details of its discovery at a news conference. Observations show it measures about 1,180-2,360km (730-1,470 miles) across, making it similar in size to Pluto.

"We think that there's evidence there is a satellite around Sedna," said Dr Mike Brown of the California Institute of Technology, US, leader of the research team that found the body. "We're hoping in the very near future to get some observations from the Hubble Space Telescope that should put that question to rest." It is three times further from the Sun than Pluto and was first seen from California's Mount Palomar Observatory.

Sedna, which is named after the Inuit goddess of the ocean, is both very red and very shiny. This combination is extremely unusual in the solar system and has baffled scientists trying to determine what it is made of. They are believed to be remnants of the formation of the Solar System and among the most primitive objects available for study

There is likely to be some debate about whether it qualifies as a true planet, but some scientists are already saying it re-defines our Solar System. Sedna, or 2003 VB12, as it was originally designated, is the most distant object yet found orbiting our Sun. It is three times further away than Pluto (average distance to the Sun is 5.9 billion km or 3.6 billion miles). It was discovered using the Mt Palomar facility in November by astronomers from the California Institute of Technology, Yale Observatory and the Gemini Observatory. Dr Brown said, in his view, the object's apparently small size suggested it should not be classified as a true planet. He suggested this "planetoid" is about half rock and half ice mixed together, but further work is needed to verify this.

Follow-up studies by the Tanagra Observatory have measured the thermal radiation coming from Sedna to determine how hot it is, and therefore provide some estimate of its size. Researchers believe that Sedna's surface temperature is about -240 degrees Celsius (-400 degrees Fahrenheit).

This estimate is uncertain but the object is likely to be between half the diameter of Pluto (2,360km or 1,470 miles) and Pluto's size; though some astronomers think it could be larger than the ninth planet itself. From the observations made so far, astronomers have determined Sedna's orbit to be a very large one. It is currently 90 times the Earth-Sun distance away (149 million km or 93 million miles), but its orbit can take it 10 times further away still.

Although Sedna could be a so-called Kuiper Belt object, its discoverers are unsure if it is as they consider it to be unlike any other object yet found. The KB contains hundreds of known objects and astronomers believe there are many more awaiting discovery. Most are small worlds of rock and ice but some could rival Pluto in size. In recent years, astronomical work has thrown up several big objects. Quaoar, found in 2002, is about 1,200km (745 miles) across. Ixion, discovered in 2001, is 1,065 km (660 miles) wide. Varuna, detected in 2000, has a diameter of approximately 900 km (560 miles).

And only in February this year, scientists picked up the object 2004 DW, which is though to be 1,800km (1,120 miles) across. The new discovery will reignite the debate about what constitutes a planet. One group of astronomers believe that Pluto is not a true planet but merely one of the largest of a vast number of minor objects in the outer Solar System. The alternative standpoint is that Pluto is a planet and those who believe that will have to classify Sedna as the 10th planet.